The present invention generally relates to an information processing arrangement and more particularly, to an optical information processing apparatus employing a semi-conductor laser such as an optical disc device or the like.
Following recent developments in the present information oriented society, optical disc devices capable of recording and reproducing information in a large capacity have been put into actual application. Such optical disc devices include those for compact discs, video discs, additional-writing type optical discs, or rewritable magneto-optical discs. Improvements in optical pick-ups or optical heads designs (which is the most essential portion of an optical disc device) through miniaturization and reduction in weight have been attempted through utilization of techniques in micro-optics owing to the necessity for high speed access and stabilization. However, the size reduction of optical heads is limited since conventional optical heads employ bulk-type optical elements (analyzers, beam splitters, prisms, lenses and other elements).
Therefore, various proposals have been recently made for reducing the size of optical heads. One proposal relates to an optical head for the magneto-optical disc device which utilizes a waveguide type differential detection device. One such device is disclosed in Institute of Electronics, Information and Communication Engineers (IEICE) Magazine, OQE 86-177, written by Sunagawa et al.
The known optical head as referred to above is schematically shown in FIG. 1.
The waveguide type differential detection device in FIG. 1 includes a PYREX.RTM. glass substrate 57, a planar waveguide layer 58 of #7059 glass formed on the substrate 57 by sputtering, a clad layer 59 of silicon nitride further formed by a plasma CVD process, and trifocal focusing grating couplers 60, 61 and 62 further formed thereon through employment of an electron beam direct drawing method and etching techniques after application of resist on the clad layer 59. The grating cycle is designed so that the focusing grating coupler 60 at the center excites the TM mode, while the focusing grating couplers 61 and 62 at the opposite sides excite the TE mode. Light emitted from a semi-conductor laser source 51 is formed into parallel light by a collimating lens 52, and introduced into said focusing grating couplers 60, 61 and 62. The zero-order diffracted light of the reflected light is transmitted through the above focusing grating couplers 60, 61 and 62, and collected on the magneto-optical disc D by a condenser lens 53. The reflected light subjected to rotation in a polarizing direction by the Kerr effect at the electro-optical disc D is directed in the reverse direction through the condenser lens 53, and formed into waveguide light of TM and TE modes by the focusing grating couplers 60, 61 and 62 so as to be led to the waveguide layer 58, and collected onto photo-detectors 63, 64, 65, 66 and 67 formed on the glass substrate 57. Thus, by calculating the differentiation of outputs of said photo-detectors 63, 64, 65, 66 and 67, the focusing error signal, tracking error signal and reproduction signal are obtained. Since the above waveguide type differential detection device is small and lightweight, with dimensions of about 5 .times. 12 mm.sup.2, a very small optical head may be constructed including the semi-conductor laser 51 and lenses 52 and 53. Therefore, the focusing and tracking functions can be executed by moving the entire optical head with an actuator (not shown) based on the above respective signals.
Incidentally, the optical head employing the conventional waveguide type differential detection device as shown in FIG. 1 adopts a push-pull practice for the detection of the tracking error signal, in which deviation of spot is detected by the intensity of .+-. primary diffracted light produced by the pit Dp or track Dt on the magneto-optical disc D. Accordingly, a problem exists where the magneto-optical disc D is inclined shifting the optical axis of the diffracted light and DC offset diffracted light producing a DC offset in the tracking error signal, thus resulting in functional instability. Moreover, a similar problem is encountered with respect to the focusing error signal employing the same diffraction light.
Apart from the above, there has also been recently proposed an integrated type optical head as described herein below (See Ura et al., Lecture No. 2P-L-15 of the Society of Applied Physics, Fall, 1985).
The integrated type optical head as shown in FIG. 2 includes a silicon substrate 31, a buffer layer 32 of SiO.sub.2 formed on the surface of the substrate 31 by a head oxidizing process, a flat plate optical waveguide layer 33 of glass formed on the buffer layer 32 by sputtering, a clad layer of silicon nitride further formed by a plasma CVD process, a focusing grating coupler 34 and two sets of focusing grating beam splitters 35 and 36 further formed through adoption of the electron beam drawing method and etching technique after application of resist on the clad layer.
The light emitted from a semi-conductor laser source 41 is introduced from the side face, into the flat plate optical waveguide layer 33 on the silicon substrate 31 and reaches the two sets of focusing grating beam splitters 35 and 36 as it advances while expanding. The light transmitted through said beam splitters 35 and 36 is altered, in its light path, to a space above the silicon substrate 31 by the coupler 34, and is collected onto the pit Dp on the optical disc D. The reflected light from the optical disc D becomes the waveguide light of the flat plate optical waveguide layer 33 by the focusing grating coupler 34 and is diffracted by the beam splitters 35 and 36.
The diffracted light is focused onto photo-detectors 37, 38, 39 and 40 formed on the silicon substrate 31.
The recorded information on disc D is read based on the amount of the reflected light from the pit Dp. In other words, the information may be detected as a sum S of the detection outputs of the photo-detectors 37, 38, 39 and 40. Meanwhile the focusing error signal is detected using the so-called Foucault method. Before the optical disc D reaches the focusing point, the light received by the photo-detectors 37 and 39 at the outer side becomes larger than the light received by the photo-detectors 38 and 40 located at the inner side due to the arrangement of the optical system. Conversely, after the focusing point, the light received by the inner photo-detectors 38 and 40 becomes larger than the light received by the photo-detectors 37 and 39. Based on the above, the sum of the outputs of the photo-detectors 37-39 and the sum of the output of the photo-detector 38-40 are calculated, whereby the focusing error signal FE is detected from the differentiation between the above two sums of the outputs. Meanwhile, the tracking error signal is detected by using the so-called push-pull practice where light received by the photo-detectors 37 and 38, and that of the photo-detectors 39 and 40 is altered by deviation in tracking. Based on this fact, the sum of the outputs of the photo-detectors 39 and 40, and that of the outputs of the photo-detectors 37 and 38 are calculated, and from the differentiation between the above two output sums, the tracking error signal TE is detected.
Since the above silicon substrate 31 is small and light weight with dimensions of 5 .times.12 mm.sup.2, a very compact optical head can be constructed including the semi-conductor laser source 41. Thus, by moving the entire optical head by an actuator (not shown) based on the focusing error signal and tracking error signal, the focusing and tracking functions may be affected. Furthermore, in order to alleviate factors such as aberration and the like, there has also conventionally been proposed a method by which light emitted from the focusing grating coupler 34 is formed into parallel light so as to be focused on the disc through a condensing lens.
However, the conventional integrated type optical head as shown in FIG. 2 adopts the push-pull practice for detecting deviations in the spot based on the strength of .+-. primary diffraction light produced by the pit Dp on the optical disc D as a method for detecting the tracking error signal. Therefore, the optical axis of the diffracted light is shifted by inclination of the optical disc D to produce DC offset in the tracking error signal, thus resulting in functional instability. Since the focusing error signal employs the same diffraction light as that used for tracking, a similar problem exists. Moreover, due to adoption of the end face coupling method for introducing the light emitted from the semi-conductor laser into the optical waveguide layer 33, the coupling efficiency is low, and there is a disadvantage in that the efficiency of the semi-conductor laser light is lowered with only 10 to 20% of the light rays in the emitted laser light becoming the waveguide light, while remaining light ray is undesirably lost.